In prior electric storage batteries for an automobile, plates usually comprise a grid composed of lead alloy containing antimony at 4 to 5 percent by weight. Since such storage batteries have an overcharging current passing therethrough to a certain degree even after they are completely charged, water in the electrolyte is electrolysed into gases so as to decrease the amount of the electrolyte. Water pouring openings which are usually provided on the tops of the battery cells cause the water to vapor therethrough due to heat from the engine of the automobile or due to heat produced when the storage batteries are charged, and therefore, the water in the electrolyte tends to be shortened. In order to maintain the performance of the storage battery, an operator should supply water regularly so as to preserve a predetermined amount of water in the electrolyte.
Of late, there may be used plates composed of lead alloy containing antimony at 2 to 3 percent by weight or containing calcium instead of antimony. In the storage battery using such plates, an overcharging current after the complete charge of the storage battery is advantageously prevented and therefore shortage of water is also prevented so that water may not be supplied. However, it is impossible to completely prevent the overcharging current from flowing through the storage battery. Thus, water is still evaporated due to heat from a modicum of overcharging current or due to heat from the engine of the automobile.
There are proposed various gas venting systems in which gases are vented out of the storage battery, but electrolyte mist which tends to be carried together with the gases is prevented from leakage out of the battery casing. One example of such gas venting systems is illustrated and described in the U.S. Pat. No. 3,772,088, in which a closure assembly for a storage battery is disclosed. In the gas venting system of the U.S. Patent, the gases from the cells are directed to a common channel and through a porous ceramic filter outside of the closure assembly. Electrolyte which is carried together with the gases are prevented from leakage by baffles and dams disposed in the channels. However, the gas venting system of this Patent is constructed to direct the gases in a horizontal manner through the closure assembly, and as a result the electrolyte mist cannot be effectively separated from the gases. This causes the substantial amount of the electrolyte mist to be leaked out of the battery cover. Furthermore, the distance between the inlet and the outlet of the gas venting system is so short that the electrolyte mist tends to be leaked together with the gases out of the storage battery.